1. Field of Invention
The present invention relates to receivers for seismic signals used in well boreholes.
2. Description of Prior Art
In sonic logging of a well seismic receivers in the form of sondes containing geophones are lowered into the well borehole on a conductive armored cable to selected depths or formations of interest. The geophones then sense the response of subsurface formations to acoustic or elastic wave energy emitted from a suitable source of seismic energy. It has been known to be important that the geophones accurately sense the seismic energy for purposes of data accuracy. Good, firm mechanical engagement between the sonde and the borehole was necessary to insure that the seismic energy was effectively coupled from the earth to the geophones. The mechanical engagement of the tool with the bore hole wall also had to be effectively releasable so that the sonde could be easily moved to selected depths and also so that the sonde could be easily retrieved.
An additional problem was that existing seismic receivers did not, so far is known, have uniform frequency response over the seismic frequency range of interest. Existing tools did not exhibit a very flat frequency response for frequencies below about one hundred hertz particularly in the horizontal axis. Causes for this problem may have resulted from insufficient clamping force, bending modes in the tool, or rocking of the tool, particularly in an axis perpendicular to that of arm members of the tool used to clamp the tool in contact with the earth formation around the well borehole, or other factors.